Well, it should be fairly obvious by the fact that the Jews are depicted as wicked and then tortured and killed in the end. I'd say that this would seem pretty anti-Semetic.
One must understand, however, the place and time of the story. It is being told by a Catholic for a Catholic audience. These views were common among Catholics of that day.
Please read the below quote (from the following link) for further info:
"True to her perfectionist, sentimental nature, the Prioress begins with a long apologetic prayer to the Virgin Mary. Her story of the martyred child resembles popular saints' stories of the day. It has a very preachy and morbid tone.
Though they appear to the modern reader as very negative aspects of this story, her reverence for chastity and her harsh judgment of the Jews are both reflections of common medieval Catholic beliefs. The violent nature of the events in the story seem to be in contradiction to a personality as sensitive as the Prioress's is supposed to be, suggesting that she may be much tougher than she wishes to reveal. She is, after all, in a position of great authority over others."